The Ebook Format Landscape in 2026
The ebook market has grown into a multi-billion dollar industry, and understanding ebook formats is essential whether you are a reader choosing between devices, an author self-publishing your first novel, or a publisher distributing content across platforms. Each format was designed with different priorities, and choosing the wrong one can result in broken layouts, limited distribution, or frustrated readers.
This guide covers every major ebook format in depth, comparing their features, compatibility, and ideal use cases so you can make an informed decision.
The Major Ebook Formats Explained
EPUB (Electronic Publication)
EPUB is the open standard maintained by the W3C (previously the IDPF). It is the most widely supported ebook format, accepted by virtually every ebook store and reading application except Amazon Kindle.
How it works: An EPUB file is essentially a ZIP archive containing XHTML content files, CSS stylesheets, images, fonts, and metadata. The .epub extension is just a renamed .zip file that follows a specific internal structure.
EPUB 2 vs EPUB 3:
| Feature | EPUB 2 | EPUB 3 |
|---|---|---|
| Content format | XHTML 1.1 | HTML5 |
| Styling | CSS 2.1 (subset) | CSS3 |
| Audio/Video | Not supported | HTML5 media elements |
| JavaScript | Not supported | Limited support |
| Fixed layout | Not supported | Supported |
| Accessibility | Basic | WCAG/ARIA integration |
| MathML | Not supported | Supported |
| Vertical text | Not supported | Supported (CJK) |
| Specification year | 2007 | 2011 (updated 2023) |
EPUB 3 is now the standard for new publications, though EPUB 2 files still work on most readers. If you are creating ebooks, always target EPUB 3 unless you have a specific reason to use the older version.
MOBI (Mobipocket)
MOBI was the original Kindle format, inherited when Amazon acquired Mobipocket in 2005. It has been officially deprecated by Amazon, which now uses the KF8/AZW3 format. However, MOBI files are still widely shared and remain compatible with older Kindle devices.
Limitations: No support for HTML5, limited CSS support, no embedded fonts, no audio or video, and a maximum file size of 64 KB per record (though records chain together). MOBI files tend to be larger than equivalent EPUB files due to less efficient compression.
AZW3 / KF8 (Kindle Format 8)
AZW3 is Amazon's current ebook format, based on HTML5 and CSS3. It offers significantly better formatting than MOBI while remaining exclusive to the Kindle ecosystem. Amazon typically wraps AZW3 content inside a MOBI container for backward compatibility.
Key features: Embedded fonts, CSS3 support (including Flexbox), fixed-layout support, SVG graphics, and better typographic control than MOBI. However, AZW3 files can only be read on Kindle devices and apps.
PDF (Portable Document Format)
PDF is a fixed-layout format, meaning every page has an exact, predetermined appearance. This makes PDF excellent for documents where layout precision matters (textbooks, graphic novels, technical manuals) but problematic for typical reading on small screens.
Ebook limitations: PDF text does not reflow to fit different screen sizes. Reading a PDF formatted for A4 paper on a 6-inch Kindle screen requires constant zooming and panning. PDFs also lack ebook-specific features like adjustable font size, dictionary lookup, and reading progress synchronization.
That said, PDF remains the preferred format for certain types of content. Our in-depth EPUB vs PDF comparison covers when each format is the better choice.
Other Formats
KFX — Amazon's latest Kindle format, succeeding AZW3. Features enhanced typesetting (better hyphenation, kerning, drop caps) and page-flip preview. Only available through Amazon's publishing pipeline.
iBooks (IBA) — Apple's proprietary format created by iBooks Author. Supports rich media (video, 3D objects, interactive widgets) but is exclusive to Apple devices. Apple discontinued iBooks Author in 2020 in favor of Pages for EPUB export.
FB2 (FictionBook) — An XML-based format popular in Russia and Eastern Europe. Supports structured metadata and semantic markup but has limited adoption outside its core regions.
DJVU — Optimized for scanned documents with high-resolution images. Not a true ebook format, but commonly used for digitized books and academic papers.
Feature Comparison Matrix
This comprehensive comparison covers the features that matter most when choosing an ebook format:
| Feature | EPUB 3 | MOBI | AZW3/KF8 | KFX | |
|---|---|---|---|---|---|
| Reflowable text | Yes | Yes | Yes | No | Yes |
| Fixed layout | Yes | No | Yes | Yes (default) | Yes |
| Custom fonts | Yes | No | Yes | Yes (embedded) | Yes |
| Audio embedding | Yes | No | No | No | No |
| Video embedding | Yes | No | No | No | No |
| JavaScript | Limited | No | No | Limited | No |
| MathML | Yes | No | Partial | Via LaTeX | No |
| SVG graphics | Yes | No | Yes | No (rasterized) | Yes |
| DRM support | Adobe, LCP | Amazon | Amazon | Adobe | Amazon |
| Max file size | No limit | ~650 MB | ~650 MB | No limit | ~650 MB |
| Accessibility | Excellent | Poor | Moderate | Variable | Moderate |
| Open standard | Yes (W3C) | No | No | Yes (ISO) | No |
Pro Tip: If you need to distribute the same content across multiple platforms, author in EPUB 3 and convert to other formats as needed. EPUB's open standard and rich feature set make it the best source format for multi-platform publishing. Use our document converter to handle format transformations.
Reader Compatibility
Understanding which devices and apps support which formats is critical for reaching your audience.
Device Compatibility
| Reader/App | EPUB | MOBI | AZW3 | KFX | |
|---|---|---|---|---|---|
| Amazon Kindle (e-ink) | No | Yes | Yes | Yes | Yes |
| Kindle App (iOS/Android) | No | Yes | Yes | Yes | Yes |
| Apple Books | Yes | No | No | Yes | No |
| Google Play Books | Yes | No | No | Yes | No |
| Kobo | Yes | Yes | No | Yes | No |
| Nook | Yes | No | No | Yes | No |
| Calibre | Yes | Yes | Yes | Yes | No |
| Readium | Yes | No | No | No | No |
| Adobe Digital Editions | Yes | No | No | Yes | No |
| Moon+ Reader (Android) | Yes | Yes | No | Yes | No |
| Libby/OverDrive | Yes | No | No | No | No |
The key takeaway: EPUB works everywhere except Kindle, and AZW3/KFX work only on Kindle. This fundamental split drives most format decisions in publishing.
Converting Between Formats
Calibre is the gold standard for ebook format conversion. It handles EPUB to MOBI, EPUB to AZW3, PDF to EPUB, and virtually every other combination. The conversion quality varies by format pair:
- EPUB to AZW3: Excellent (structurally similar)
- EPUB to MOBI: Good (some features lost)
- PDF to EPUB: Variable (depends on PDF complexity)
- EPUB to PDF: Good (but loses reflowability)
For quick conversions without installing software, our EPUB converter handles the most common format transformations. If you need to convert between EPUB and MOBI specifically, our guide on converting EPUB to MOBI covers the process and pitfalls in detail.
Reflowable vs Fixed Layout
This is the most important architectural distinction in ebook formats, and it determines how your content adapts to different screens and reading preferences.
Reflowable Layout
Reflowable ebooks adapt their layout to the reader's screen size, font size preference, and reading orientation. The text wraps, images resize, and the pagination adjusts automatically.
Best for:
- Novels and long-form prose
- Non-fiction books with running text
- Content that will be read on multiple screen sizes
- Books where readers want to adjust font size and line spacing
Limitations:
- No control over exact page layout
- Complex layouts (multi-column, sidebar, pullquote) are difficult
- Page numbers are meaningless (change with every setting adjustment)
- Tables and figures may break across pages unpredictably
Fixed Layout
Fixed-layout ebooks present each page as a designed canvas with exact positioning of text, images, and graphics. The reader can zoom but cannot change the layout.
Best for:
- Children's picture books
- Graphic novels and comics
- Textbooks with complex layouts
- Cookbooks with full-page photography
- Art books and portfolios
- Technical manuals with precise diagrams
Limitations:
- Poor experience on small screens (requires zooming)
- No font size adjustment
- Larger file sizes
- Reduced accessibility (screen readers struggle)
Pro Tip: If your book has mostly text with occasional images, use reflowable layout. Only choose fixed layout when the visual design of every page is essential to the reading experience, like in a graphic novel or photography book.
Self-Publishing Format Requirements
Each major ebook platform has specific format requirements for authors uploading their books.
Amazon Kindle Direct Publishing (KDP)
Accepted formats: EPUB (preferred), DOCX, KPF (Kindle Package Format), PDF (discouraged)
Amazon converts your uploaded file to its proprietary formats internally. Starting in 2023, Amazon recommends uploading EPUB files for the best conversion quality. The older workflow of uploading MOBI is now deprecated.
Key requirements:
- Cover image: JPEG or TIFF, minimum 625 x 1000 px, recommended 2560 x 1600 px
- Interior images: RGB color mode, minimum 300 DPI for print
- Maximum file size: 650 MB
- Table of contents: Required (HTML-based, not NCX-only)
Apple Books
Accepted formats: EPUB 3 (preferred), EPUB 2
Apple has strict EPUB validation requirements. Files must pass epubcheck with zero errors. Apple Books supports the widest range of EPUB 3 features, including embedded audio, video, and JavaScript widgets.
Google Play Books
Accepted formats: EPUB, PDF
Google accepts both EPUB and PDF but recommends EPUB for reflowable content. PDF books are listed as "scanned pages" and receive less favorable placement in search results.
Kobo
Accepted formats: EPUB 3, EPUB 2
Kobo is fully EPUB-centric and has excellent support for EPUB 3 features, including embedded fonts, advanced CSS, and fixed layout.
Format Requirements Summary
| Platform | Preferred Format | Cover Min Size | Max File | DRM |
|---|---|---|---|---|
| Amazon KDP | EPUB | 2560 x 1600 px | 650 MB | Amazon (automatic) |
| Apple Books | EPUB 3 | 1400 x 1873 px | 2 GB | FairPlay (optional) |
| Google Play | EPUB | 1600 x 2400 px | 2 GB | Adobe (optional) |
| Kobo | EPUB | 1400 x 1873 px | 700 MB | Kobo (optional) |
| Smashwords | EPUB | 1600 x 2400 px | 5 MB | None |
| Draft2Digital | EPUB | 1600 x 2400 px | 200 MB | Retailer-specific |
Creating Ebooks: Recommended Workflows
For Fiction Authors
The simplest workflow for a novel or short story collection:
- Write in a word processor (Word, Google Docs, Scrivener)
- Format with proper heading styles (Chapter Title = Heading 1)
- Export or convert to EPUB using our document converter
- Validate with epubcheck
- Upload to distribution platforms
Our tutorial on creating EPUBs from Word documents walks through this process step by step.
For Non-Fiction Authors
Non-fiction books with images, tables, and complex formatting need more attention:
- Author in Word or InDesign with consistent styles
- Optimize images (72-150 DPI for screen, JPEG for photos, PNG for diagrams)
- Convert to EPUB 3 with embedded fonts
- Test on multiple readers (Kindle, Apple Books, Kobo)
- Create separate PDF version for print if needed
For Technical Authors
Technical books with code listings, math equations, and diagrams:
- Author in Markdown, AsciiDoc, or LaTeX
- Use Pandoc or a documentation generator to produce EPUB
- Ensure code blocks render correctly with monospaced fonts
- Test MathML rendering on target platforms
- Consider a separate PDF version for precision formatting
DRM: Digital Rights Management
DRM protection is one of the most debated topics in ebook publishing. Here is what you need to know:
Types of Ebook DRM
- Amazon DRM: Proprietary, tied to Amazon accounts. Prevents reading on non-Kindle apps.
- Adobe DRM (ADEPT): Used by most non-Amazon retailers. Requires Adobe Digital Editions or compatible apps.
- Apple FairPlay: Used by Apple Books. Tied to Apple ID.
- LCP (Licensed Content Protection): Open-standard DRM for EPUB. Growing adoption (Kobo, libraries).
- Social DRM (watermarking): Embeds buyer information without restricting access. No compatibility issues.
Should You Use DRM?
Most self-publishing experts recommend against DRM for independent authors. DRM does not prevent piracy (DRM removal tools are widely available) but does frustrate legitimate buyers who want to read on multiple devices. Watermarking (social DRM) provides accountability without compatibility friction.
Major publishers continue to use DRM largely due to contractual obligations with authors and agents, not because it effectively prevents unauthorized copying.
Optimizing Ebook File Size
Large ebook files download slowly, consume device storage, and may exceed platform upload limits. Here are strategies for keeping file sizes manageable:
Image Optimization
Images are almost always the largest component of an ebook. Optimize them before embedding:
- JPEG quality 70-80% for photographs (visually indistinguishable from higher quality on e-ink screens)
- PNG only for images requiring transparency or sharp edges (logos, diagrams, screenshots)
- Maximum dimensions: 1600 px on the longest side for full-page images; smaller for inline images
- Resolution: 72-150 DPI (e-ink screens are typically 300 PPI but display at reduced resolution)
Use our image compressor to reduce image file sizes before embedding them in your ebook. For a deeper dive, our guide on lossless vs lossy compression explains the tradeoffs.
Font Optimization
Embedded fonts add 50-500 KB each to your ebook. Subset fonts to include only the characters actually used in your text. Tools like pyftsubset (part of fonttools) can reduce a 200 KB font file to under 30 KB.
Content Structure
- Remove unnecessary whitespace and formatting from the XHTML source
- Compress internal images within the EPUB ZIP archive
- Split long chapters into multiple XHTML files (improves rendering performance)
Accessibility Considerations
Ebook accessibility is not just a legal requirement (it is increasingly mandated by law in the EU and other regions) — it also reaches a wider audience and improves the reading experience for everyone.
Accessibility by Format
- EPUB 3: The best accessibility support through ARIA roles, semantic HTML5, alternative text, and reading order metadata. Fully compatible with screen readers.
- PDF: Variable accessibility. Tagged PDFs with proper reading order can be accessible, but most PDFs are not well-tagged. Our PDF accessibility guide covers this in depth.
- AZW3/KFX: Moderate accessibility. Amazon's VoiceView screen reader works with Kindle, but the closed ecosystem limits third-party assistive technology.
- MOBI: Poor accessibility. The format predates modern accessibility standards.
Key Accessibility Practices
- Add
altattributes to all images - Use semantic heading hierarchy (H1 > H2 > H3)
- Provide text descriptions for complex figures and charts
- Ensure sufficient color contrast in fixed-layout designs
- Include a navigable table of contents
- Add
langattributes for multilingual content - Test with screen readers (VoiceOver on Apple, TalkBack on Android)
Making Your Choice
For most authors and publishers in 2026, the decision comes down to this:
- EPUB 3 is the universal format. Author in EPUB and convert to other formats when needed.
- Use PDF only for content where exact visual layout is essential and small-screen reading is not a priority.
- Do not author in MOBI. It is deprecated. Upload EPUB to Amazon and let them convert.
- Test on real devices. Emulators and previewers do not catch all rendering issues.
If you need to convert between ebook formats, our document converter and EPUB converter handle the most common transformations. And for a focused comparison of the two most popular formats, our EPUB vs PDF guide provides detailed analysis of when each format excels.
